An Arrayed Waveguide Grating (AWG) is a device commonly used as a frequency or wavelength demultiplexer and can be viewed as a spatially dispersive lens. For example, an image received at the input of the AWG will be projected onto an output plane similar to a conventional lens, but the position of the output image on the output plane is wavelength dependent. Accordingly, the position of the output image changes as the wavelength of the input image changes.
FIG. 1 illustrates the components of a conventional AWG 100. As shown in FIG. 1, an input image 114 is projected from an input waveguide 102 into an input free-space propagation region 104. The image expands, or diffracts, within the input free-space propagation region 104. A waveguide array 106 is disposed at an opposite end of the input free-space propagation region 104 and collects the expanded field. Waveguide array 106 comprises a series of quasi-parallel waveguides where the length of each waveguide increases by a constant and specified amount from its inner neighbor. That is, starting from the innermost shortest waveguide, each subsequent waveguide is longer by a specified amount.
The collected field is received by the waveguide array 106 and projected into an output free-space propagation region 108. The image output from waveguide array 106 propagates through the output free-space propagation region 108 onto the output image plane 110 containing output waveguides 112. Due to the phase curvature and phase tilt induced by the waveguide array 106, the field (image) is refocused on to an output image plane 110. The output image 116 is initially received at the right side 110a of the output image plane 110 and scans from the right side 110a to the left side 110b as indicated by arrow 118. The position of the output image 116 on the output image plane 110 may be described as a function of wavelength, and the movement of the image 116 across the output image plane 110 of the AWG 100 is generally referred to as the scanning property of the AWG. For conventional AWGs, the position of the output image 116 on the output plane 110 is generally a linear function of the optical frequency.
The output waveguides 112 collect the image as it scans across the output image plane 110. Typically, the input waveguides 102 and the output waveguides 112 have the same dimensions such that, in an ideal AWG, the output image will substantially match the mode profile of the output waveguide. Accordingly, when the output image 116 is centered on an output waveguide, the transmission response from input to output is nearly 100% (i.e. unity). Because the position of the output image 116 changes with frequency, each of the output waveguides 112 will collect light at different frequencies thereby functioning as an optical frequency demultiplexer. Each of the output waveguides 112 collects the maximum amount of energy when the frequency of the output image 116 centers the output image 116 on an output waveguide 112. The amount of energy collected by an output waveguide 112 is reduces as the frequency of the output image 116 varies from the center frequency of the output waveguide 112.
Conventional AWGs in which the image position on the output image plane 110 is a linear function of frequency have Gaussian shaped frequency responses. Put another way, the amount of light collected by an output waveguide varies with the frequency of the light such that a maximum of light is collected at a single (center) frequency and is gradually reduced as the frequency varies farther and farther from the center frequency, e.g., the response is parabolic with an apex of the parabola at a single frequency. The details of the Gaussian shaped response of the AWG, such as its frequency bandwidth, are determined by the shape and refractive indices of the input and output waveguides 102, 112 as well as on the design of the entire AWG 100. The Gaussian responses of conventional AWGs 100 result in significant losses for frequencies outside of the center frequencies of the output waveguides which are not ideal in communication systems. Accordingly, an AWG with an improved response is desirable.